Container cranes are used to handle freight containers and especially to transfer containers between transport modes at container terminals, freight harbours and the like. Standard shipping containers are used to transport a great and growing volume of freight around the world. Transshipment is a critical function in freight handling. Transshipment may occur at each point of transfer and there is usually a tremendous number of containers that must be unloaded, transferred to a temporary stack, and later loaded on to another ship, back onto the same ship or loaded instead onto another form of transport. Loading and unloading containers to and from a ship takes a great deal of time. The development of automated cranes has improved loading and unloading and made the productivity more predictable, and also eliminated many situations in which port workers have been exposed to danger and injury.
For accurate handling of containers the control systems that regulate the picking up and landing of containers must be calibrated. This may comprise calibrating sub systems of the crane control systems. For example on gantry cranes or ship-to-shore cranes (STS) that run on rails, a somewhat random error that may occur is caused by changes in one or more wheel positions on a gantry rail, which may cause a skew error. Other errors may arise from subsidence in or damage to the area the containers stand upon, so that the position of a landing slot for a container may change. In addition, when optical sensor equipment or position encoder sensors are repaired or moved a re-calibration is necessary.
It is estimated that with today's manual procedures it may take about 4-8 hours per crane to perform a LPS (Load Position Sensor), TPS (Target Position Sensor) and co-calibration. A LPS subsystem finds the position of the load (container or empty spreader) during lifting, handling, and a TPS subsystem finds the position of a target landing place on a ground slot or on a vehicle, as well as mapping positions of other containers, container stacks etc in the vicinity of a target. In addition, depending on how much time is available, an estimated 1-4 hours may be spent on stacking tests and parameter fine-tuning. These are average estimates for a block of containers, which is a given stacking area of eg between two adjacent cranes, when the block has been emptied and taken out of production. If calibration is to be performed on a crane in a block that is in production it often takes more time than that because the procedure is interrupted and has to start over several times. In addition it is often not allowed, on safety grounds, for a maintenance person to work alone in a block of containers.
The error in measurement may come from any of many sources such as: inclination in gantry rail; curves in gantry rail causing skew in crane position; wheel position on gantry rail causing offsets in trolley direction; wheel position on gantry rail causing skew in crane position; gantry positioning error (synchronization offset); twisted trolley girder profile causing error in measurement angle; skew of trolley platform on trolley rail; LPS system calibration error; TPS system calibration error.
Some errors such as TPS system calibration error tend to be constant through a given block of containers. Other errors such as gantry rail inclination and direction depend on gantry position and may thus differ from bay to bay within a given block. Error in gantry inclination also twists the trolley girder, which makes the error different from one row of containers to another in the same block.